The unconscious Professor Xavier has been kidnapped by Factor Three while the X-Men were away on a mission. However, even worse, the villains also wrecked their mutant-tracking device Cerebro. Whereas the Beast starts trying to repair the machine, Cyclops reasons that this act confirms their theory that Factor Three is composed of evil mutants - why else would they destroy Cerebro.
Handing his colleague some of the tools required for the repairs, Angel fears that if Factor Three succeed in somehow gaining access to Xavier’s knowledge, they might eventually reach their goal of world conquest in no time. Whatever the case, Beast says that, with the manual-labor of the work done, Cerebro’s wiring is all that remains to be fixed, though that will take several hours.
Knowing that the Bobby and Warren can’t be of help, Scott tells them to turn in. Warren takes offense by this, wanting to pull his weight, but Jean interrupts him, saying that Scott is right. At least some of them should have gotten some rest in case that Factor Three attacks again. Right after the three X-Men left the room, Scott worries that he might have been too harsh with Warren, though Hank reassures him that it was the right decision – after all, what matters most now is to rescue the Professor. However, Scott still decides to make amends and goes after Warren to apologize for pulling rank on him. While the two X-Men make up, Jean thinks to herself that Scott’s selflessness is possibly the reason why she loves him so much.
(about an hour later)
Jean returns to her room at Metro College, where her roommate, Carole, informs her that Ted Roberts has left her an urgent message to call him back. Jean knows that Ted suspects her to be Marvel Girl, and she thinks that he must have heard of the X-Men’s battle with the Juggernaut and is concerned about her well-being. Still unsure what to say when he asks her about her secret identity, Jean makes the phone call. A rather desperate Ted picks up the phone, his head bandaged.
He tells Jean that he is at his brother, Ralph, has been kidnapped and that he wants her to get the X-Men to rescue him. Jean keeps up her pretense, asking why he would think that she could reach the X-Men, but Ted says he has no time to explain and hangs up. Knowing Ted good enough to know that he wouldn’t pull such a stunt just to get her to reveal her secret ID, Jean decides that she has no other choice but to help him.
She calls Angel and Iceman at Xavier’s school, neither one having been able to find some sleep, and asks them to pick her up on the way to the research labs of Ralph Roberts. The two X-Men decide not to disturb the Beast and Cyclops’ repairs of Cerebro, and board a small plane. After a quick stop at a hidden location near the campus of Metro College to pick up Marvel Girl, the flight continues to Long Island, where the main compound of Roberts Research Inc. is located. During the flight, the X-Men recall that Ralph was on the TV the other day, talking about some new government project and she wonders if he too could have been captured by Factor Three like Xavier.
The three X-Men are greeted by an unnerved Ted Roberts, who thanks them for coming, although he would have preferred the entire team for the attempt to rescue his brother from the center of the Earth. The X-Men having agreed on Jean remaining silent to no further compromise her identity, it’s Angel and Iceman who ask Ted to slow down and start the story from the start. He informs the three mutants that they have to act quickly, for if Ralph refuses to do as Tyrannus asks of him, he will surely be killed.
Angel answers that they have heard rumors about some underground ruler names Tyrannus, but so far he had believed it to be a legend. Ted says that there is no more doubt of Tyrannus‘ existence, and continues his narration. His brother had been experimenting with cobalt alloys, and last week he made a scientific breakthrough.
(flashback, a few hours ago)
Ralph has invited Ted to his labs to show his brother his latest discovery – a cobalt-alloy so hard that it couldn’t even be penetrated by the most powerful laser beam. The new alloy is many times stronger than steel and its melting point is twice that of normal cobalt. Ted understands the worth of this new alloy – a ship made of it could travel to the Earth’s core without melting.
Outside the building, a ship emerges from the Earth, carrying Tyrannus and a group of subterrans. They quickly defeat any security guards of the plant, and soon make their way to the labs. By the time Ted and Ralph hear the shooting outside, it’s already too late. Tyrannus blasts the door to the lab open, and orders his followers to capture Ralph alive, for he wants him to become his slave. Ted tried to intervene, but is easily knocked over by Tyrannus. The last thing Ted hears before passing out is Tyrannus gloating that he will use Ralph’s discovery to defeat his hated arch-enemy – the Mole Man.
Ted concludes his tale by explaining that he was later found by the guards, who woke him up and treated his head injury. Actually, he blames himself for allowing his brother to be captured, but Angel reassures him that there wasn’t anything he could have done. These subterrans are much tougher than average humans for they have to survive being underground all the time, so someone like Ted hardly has a chance against them. The X-Men might be able to fight them, Warren adds, if only they had a way to follow Ralph’s captors underground. In that, for once, Ted can help, as he leads the trio of X-Men into another room of the complex, holding an atomic-powered Earth-Borer craft, already coated with plates of the new super-cobalt. Next, the groups check out the pit outside the lab complex, from where Tyrannus and his followers came. While Tyrannus and his minions filled in most of the tunnel after them, Ted thinks that it could be opened again with the Earth-Borer.
As the Earth-Borer has only room for a crew of three, Angel easily convinces Ted to stay behind as their contact, while the three X-Men board the craft. After quickly familiarizing themselves with the controls, they take off. First the steer the earth-Borer out of the lab towards the pit, and then it goes downwards, the Earth-Borer’s in-built lasers blasting through dirt and rocks as they go. For long minutes the Earth-Borer moves downwards at a steady deliberate rate, until at about twenty miles underneath the surface, the three X-Men find a large cavern opening up in front of them. They are so startled by the unexpected sight that they fail to stop the craft before they hit a large stalagmite. Fortunately neither of the X-Men has come to harm and, as they leave the craft and examine it from the outside, they find that the Earth-Borer is only slightly damaged.
Checking out their surroundings, the X-Men find that the huge cavern goes on for miles, unsuspected by the Earth above, and is bathed in a strange eerie, emerald glow. Still wondering about the origins of this place, they are startled when a voice behind them answers their questions – they are on the outer edge of Earth’s mantle, a layer composed of magnesium and silicates. The voice says it hopes the X-Men like the decor... for they will never leave. The three mutants turn around to see a figure standing on a boulder, who they assume to be Tyrannus. Hearing that name, the stranger gets even angrier and fires his weapon at a rock right before the Angel, disintegrating it. While trying to get to safety, the X-Men wonder who their mysterious enemy is, who keeps firing his ray gun at them.
Finally introducing himself as the Mole Man, the foe fails to notice the Angel circling around and attacking him from the other side. While he breaks away from the winged X-Man’s grip, the Mole Man drops his weapon and calls out for his subjects to aid him against the intruders. Four of his moloids emerge from the nearby caves, but Marvel Girl, Iceman and Angel defeat them with ease. Still, they served as a long enough distraction for the Mole Man to run away. The three mutants take after him, and Jean reminds the other two to be careful, as they are on the Mole Man’s home ground.
Her warning falls on deaf ears, though, as just around the next corner the X-Men have lost sight of their foe and instead find themselves by an underground river with strange mists rising from it. A few breaths are enough to make all three mutants forget who they are or from where they came. From a safe distance, the Mole Man gloats – it is exactly as he predicted. The mutants have fallen prey to the magic mists of the river Lethe.
Meanwhile, in a nearby castle, Tyrannus is overseeing Ralph Roberts‘ work in one of his labs. There’s a giant robot, about thirty feet tall, resembling a Roman gladiator equipped with a shield and a sword, and Roberts is forced to coat the robot with his new super-cobalt alloy. Tyrannus explains that his archenemy, the Mole Man, built a similar robot made of diamond, giving him an advantage in their long-standing feud. Ralph understands that Tyrannus needed his super-cobalt, as it’s one of the rare substances harder than diamond. However, he warns the tyrant that the material has yet to be tested over long periods.